Summary

Without water, no form of life as we know it could exist. There is therefore great interest in detecting liquid water on other planets of our Solar System. Landforms such as dry river valleys and lakes show that liquid water must have been present on Mars in the past (1). Nowadays, small amounts of gaseous water exist in the martian atmosphere, and some water ice is found on the planet's surface. Water droplets were seen condensing onto the Phoenix lander (2), and there may be reoccurring water activity on slopes during the martian summer (3). However, stable bodies of liquid water have not been found on Mars. On page 490 of this issue, Orosei et al. (4) report an analysis of radar data from the Mars Express mission that shows the existence of stable liquid water below 1.5 km of ice, close to the martian south pole.